Concentrating collectors with ratios in the magnitude of 50:1 are commercially available. Such collectors are shown as early as Abbot U.S. Pat. No. 1,855,815. These concentrating collectors are used in higher temperature applications, i.e., fluid output ranges from 250 degrees F. to 500 degrees F. and above. The concentrating ratio is defined as the ratio of the reflector "opening" measured along a chordal line transverse the aperature divided by the outside diameter of the absorber tube.
Fabrication inaccuracies causing variations in the angle of the reflector surface are almost impossible to avoid within reasonable manufacturing cost objectives. These small variations result in diffused rays that are visible as a secondary "band" of light when they impinge against a thin opaque substrate which is co-planer with a line central to the reflector surface and the focal line. A narrow intense white band of light at the focal line is visible as polarized light that is focused correctly. In addition, accumulated errors in fabrication of the reflector surface show as a less intense and wider (vertical height) band of light, usually above and below the focused light rays.